The present invention relates to well cleaning methods and apparatus. In particular, but not exclusively, the present invention relates to a method of drilling and cleaning a wellbore.
In the drilling and production of oil and gas wells, it is typical to provide a drill string which is terminated by a drill bit. The drill string is rotated to remove formation ahead of the drill bit, to drill and thus form a wellbore, and to increase the depth of the well. Drilling mud or other fluid is pumped through the drill string to cool the drill bit, and to aid the passage of drill cuttings from the base of the well to the surface, via an annulus formed between the drill string and the wall of the wellbore.
At fixed intervals, the drill bit is removed from the wellbore and a casing comprising lengths of tubular casing sections coupled together end-to-end is run into the drilled wellbore and cemented in place. A smaller dimension drill bit is then inserted through the cased wellbore, to drill through the formation below the cased portion, to thereby extend the depth of the well. A smaller diameter casing is then installed in the extended portion of the wellbore and also cemented in place. If required, a liner comprising similar tubular sections coupled together end-to-end may be installed in the well, coupled to and extending from the final casing section. Once the desired full depth has been achieved, the drill string is removed from the well and then a work string is run-in to clean the well. Once the well has been cleaned out, the walls of the tubular members forming the casing/liner are free of debris so that when screens, packers, gravel pack assemblies, liner hangers or other completion equipment is inserted into the well, an efficient seal can be achieved between these devices and the casing/liner wall.
The step of cleaning the wellbore is usually achieved by inserting a work string containing dedicated well clean-up or cleaning tools. Typical well cleaning tools known for use in this environment include scrapers, wipers and/or brushes which are held against the internal wall of the casing/liner, to clean away debris as the tool is run-in and then pulled out of the wellbore. While this process is effective in cleaning the wellbore, it adds a significant amount of time to the job of preparing the well for production, since a separate well clean-up string requires to be run in the bore after the drill string has been removed. Additionally, the speed at which the string can be run-in and pulled from the well is relatively low, due to the required constant frictional contact between the cleaning members and the casing/liner wall.
Additionally, the formation in the wellbore is left exposed during the clean-up operation, and there are known disadvantages in leaving a formation exposed between drilling and completion of a well.
One known type of cleaning apparatus is disclosed in UK Patent Publication No. 2327963 (Appleton et al). GB2327963 describes a work string combining a packer with a scraper. The scraper is used to clean the casing ahead of the packer so that the packer can be set against a debris free casing. While this method removes the requirement of running a separate clean-up string before the packer is inserted, such a string is limited in that the scraper can only clean a fixed distance ahead of the packer and, as a result, only a portion of the casing is cleaned. Also, the debris removed is pushed into the wellbore.